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05-28-2012, 01:14 PM   #1
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K-7 two shots with same exposure, but one was very underexposed

I took two shots with my DA35 Macro using the exact same exposure (ISO 200, f/4.0, 1/125) and one of them is extremely underexposed and the other is not. Any idea what might have happened here?

I attached the two shots as an example, but this happened three different times yesterday. I was in Av mode with ISO set to 200.

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PENTAX K-7  Photo 
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05-28-2012, 01:41 PM   #2
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check if you have accidentally left bracketing on....
05-28-2012, 02:34 PM   #3
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nope. the exif data matches. the two exposures should be identical.
05-28-2012, 02:45 PM   #4
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Shot in a burst or individually?

05-28-2012, 02:54 PM   #5
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This happens with my Kx but not often. Does it ever happen with another lens ?
It only happens with my Tamron 90 macro and 9 out of 10 is shooting a bright subject. Same thing the Exif is the same but pic is under exposed.
Never figured it out.
Good luck.
05-28-2012, 03:24 PM   #6
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The shots were made individually, not in burst mode and bracketing was turned off. This is the first time I've seen the problem, so it hasn't shown up with any other lens. @bobpur, that sounds like the same problem except that the subject wasn't bright. I can't seem to make it happen again today.
05-28-2012, 03:40 PM   #7
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The aperture not stopping down (which I've had happen) makes much more sense than it stopping down too far, but I guess it could happen -- if you pump up the dark one, does it have greater depth of field like it was shot at f/22 or something? Otherwise, it must be the shutter or a very fast eclipse.
05-28-2012, 03:41 PM   #8
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Is the aperture on the lens sticking perhaps?

05-28-2012, 03:48 PM   #9
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It's hard to tell, but the DOF might be greater in the darker photo. It seems very strange that the aperture would close too much. Usually if it sticks, it stays open for an over-exposed shot. It's also only a 2 year old lens. The electronics in the camera could have set the wrong shutter speed, or sensor gain, I guess, but there's not much I can do to fix that. Maybe vonBaloney is right about the eclipse.
05-28-2012, 04:27 PM   #10
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Had that happen to my K7 as well few years ago.
Sent it in and they had to replace the motherboard. Haven't had the problem again since.
05-28-2012, 04:28 PM   #11
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QuoteOriginally posted by br.davidson Quote
It's hard to tell, but the DOF might be greater in the darker photo. It seems very strange that the aperture would close too much. Usually if it sticks, it stays open for an over-exposed shot.
Yes, mechanically it doesn't make sense that it could accidentally go too far in defiance of the spring pushing it open. Which leaves either the camera setting the wrong shutter speed or aperture. Maybe a dirty contact is messing up communication? (But then the EXIF data would probably be screwy as well.) Try cleaning contacts on lens and body anyway. Weird.
05-28-2012, 04:47 PM   #12
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Thanks for the help, everyone.
05-29-2012, 08:10 AM   #13
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Hi

Went back and revisited my problem, seems that the Aperture lever on the lenses travel in an Arc where as the Camera
arm travels along a straight line. It is possible that if the lens lever is a bit too far away ( bent towards the center of the lens )from the camera it can slip off of
the camera arm, and seeing that the lens spring closes the aperture it could cause the problem.
The Cam would still register the right Exif but mechanically the lens would be closed down.
I only have the one lens doing it so I checked with a caliper and found it was slightly off compared to a Pentax M lens.
Bent it to the same distance and will test it out.
Makes sense that it would not happen at larger Apertures as the arm would not move as much.
05-29-2012, 09:24 AM   #14
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Great investigating bobpur! That sounds very plausible. I'll check it out myself tonight.
06-03-2012, 07:15 PM   #15
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Results

I used the lens that was giving me trouble, up and down and all around and it never bugged. On a Kx, K100 and K10 so
the fix probably worked or the gremlin took the week off.
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